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working stress design method

working stress design method also called the allowable stress design method (WSD). used from the 1900s until the early of 1960s to design reinforced concrete structures. in this method the dead and live load to be supported called working or service load estimated. then the member proportioned so the stress will not exceed a permissible value. in this method steel and concrete acts together and concrete assumed to be elastic. where the stress in compression varies linearly from the extreme fiber to the neutral axis as shown in figure 1.

Figure 1

Limitation of WSD:

  • The assumption of linear elastic behavior of structural member is unrealistic due to long term stress such as creep and shrinkage
  • uncertainties of different applied load are not well defined and considered in WSD method.
  • the factor of safety is not well defined in this method. the factor of safety against collapse is unknown for this method
  • this method does not distinguish between dead load and live load. no consideration for the different factor of safety for different loads.
  • this method does not take into account the possibility of accidental load increase or overloading 


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